Professor Gradwell, of King's College London, explained: "Learning is a bit impaired at 8000ft so performance of recently learned tasks is a bit impaired at 8000ft. You don't want to make the biggest decision of your life."
However, if you are well-practised in a task, it is unlikely your performance will be affected.
Prof Gradwell, the UK's first full-time professor of aviation medicine, said: "If you regularly do a crossword, you are going to be fine. But if it is something that is new to you, you are probably not going to do it as well."
This explains why commercial pilots' thinking is not impaired. They get used to carrying out tasks in a simulator at ground level before they might have to carry them out during a flight.
Prof Gradwell said his advice could become even more pertinent if internet access becomes widespread on planes, meaning businessmen and women are more likely to use flying time to work.
"Those looking forward to having emails in flight might want to think twice about sending that email to the bank manager," he said.
The former RAF medic also insisted that most people's fears of deep vein thrombosis are groundless.
The condition, dubbed 'economy class syndrome', has caused anxiety among holidaymakers, business people and frequent fliers.
But Prof Gradwell said it's extremely rare, other than in those who have late-stage cancer or another medical condition that predisposes them to blood clots.
The evidence that compression stockings will help prevent DVT in healthy people is inconclusive.
Passengers should, however, stay hydrated and exercise their legs.
"No matter how small your seat, you can always flex your seat, raise and lower your feet and exercise your calves," he said.
"Flying is safe. Millions of people do it every year, without any issues at all."
- DAILY MAIL
